The present invention relates to seal assemblies and in particular to seal assemblies utilising dry gas seals.
In seal assemblies for, for example, gas compressors, it is common practice to provide seal assemblies on either side of the impellor shaft, to seal the compressor chamber on the suction and discharge side. Such seal assemblies typically include a pair of gas seals spaced axially of one another to define a chamber therebetween.
Leakage across the inboard gas seal is collected in the chamber defined between the two gas seals and, hitherto, has been ducted away to a flare stack or atmospheric vent.
Leakage under dynamic conditions is typically from 20 to 100 standard liters per minute, depending on the suction pressure of the compressor.
Because the product gas may be contaminated, it is conventional practice to provide a supply of filtered product gas to the product side of the inboard seal. Product gas is supplied either from a higher pressure stage of the compressor or alternative supply and is fed through a filter system and delivered back to the product side of the inboard seal. In order to ensure flow of the filtered gas, the product sides of the inboard seals are run at suction pressure. The amount of filtered gas delivered to the seals is in excess of the amount of leakage across the seal, so that the excess filtered gas will flow into the compressor chamber, preventing unfiltered product gas therefrom, from coming into contact with the inboard seal.
Such systems function well under dynamic conditions when the compressor is working. However, under static conditions, the pressures on the suction and discharge of the compressor even out. Under such conditions, there is no pressure differential to cause the filtered gas to be delivered to the product side of the inboard seal and pumping of the filtered gas is required. Under static conditions, leakage across the inboard seal will be reduced to of the order of 25% of the dynamic leakage.
The present invention provides a system in which leaking across the inboard seal gas is recirculated, thereby avoiding the need to flare or vent the gas and the consequent loss. An additional benefit is the substantial reduction of emissions into the atmosphere.
In accordance with one aspect of the present invention, a seal assembly for sealing a pressurised gaseous product comprises a pair of seals spaced axially to provide a leakage collection chamber therebetween, an inboard seal being a gas seal which is disposed between the sealed gaseous product and the chamber, means being provided to supply clean gas to an inlet opening to the side of the inboard seal exposed to the gaseous product, the chamber defined between the seals being connected to a reservoir, the reservoir being connected back to the inlet via a pressure intensifier and means being provided to deliver additional clean gas to the reservoir when pressure in the reservoir falls below a predetermined value.
With the seal assembly described above, clean gas leaking across the inboard seal is collected in the reservoir and is recycled back to the inboard side of the inboard seal, thus avoiding the need to flare or vent off any leakage gas.
Under dynamic conditions, the pressure intensifier is controlled to maintain the pressure in the reservoir between predetermined limits.
Where the system is connected to a flare stack, the predetermined limits will be up to the flare stack back pressure, preferably from 5% to 95% or more preferably from 30% to 95% of the flare stack pressure.
Where the system is vented to atmosphere, the upper predetermined limit is set by the spring load pressure of a non-return valve between the reservoir and atmospheric vent. Preferred limits will be from 5% to 95% or more preferably from 30% to 95% of the spring load pressure.
The flare stack back pressure or spring loaded pressure may be up to 5 barg above atmospheric pressure but is more usually from 1 to 2 barg above atmospheric pressure.
Under static conditions, the pressure intensifier is controlled to provide a flow rate at the inlet to the inboard side of the inboard seal in excess of leakage across the inboard seal, the additional clean gas required to do this, being delivered to the reservoir from a supply of clean gas.
Additional clean gas is introduced into the reservoir to prevent a vacuum forming therein. Preferably additional clean gas will be introduced into the reservoir when the pressure in the reservoir falls below 30% of the flare stack back pressure or the spring load pressure, more preferably additional clean gas is introduced when pressure in the reservoir falls below 0.3 barg.
According to a preferred embodiment of the invention both the inboard and outboard seals defining the leakage collection chamber are gas seals. However, other forms of seals may be used on the outboard side of the chamber, for example segmented carbon rings, close clearance bushes or labrynth seals. Moreover, additional seals may be provided between the inboard seal and the product chamber.